This invention relates to the analysis of organic coating materials on inorganic substrates, and more particularly to a method of analyzing molecular layers of amine-type coupling agents on glass spheres and other discrete particles.
The present invention, while of general application, is particularly well-suited for use in the qualitative and quantitative analysis of amino functional group silane coatings on small glass spheres. Coatings of this type are commonly applied to the spheres to enhance the bonding of the inorganic glass substrate to resinous materials. The substrate is thus made available for a variety of uses. A typical use involves the providing of retro-reflectance for highways. In cases in which the spheres are to be used as fillers in nylon, polyphenylene oxide, paints, alkyds, epoxies, chlorinated rubber, or other polymers, for example, the coating improves the dispersibility of the glass in the resin and provides positive adhesion therebetween.
The amount of coupling agent applied to the individual glass spheres must be carefully controlled. The spheres are unique because of their nonporous surface and small size, small glass spheres typically ranging between 6 and 800 microns in diameter depending on their final application. Each sphere must be completely coated with at least one molecular layer of coupling agent to provide a satisfactory bond. If the thickness of the coating exceeds about five molecular layers, however, there is too great a quantity of coupling agent to provide an effective bond, manifesting itself in less satisfactory physical properties of the end product. Additionally, the excess coupling material is wasted when more than five layers are present.
Heretofore, difficulties have been encountered in analyzing these small amounts of coupling agents on glass spheres and other nonporous inorganic substrates. Conventional analytical techniques have proved deficient particularly in the measurement of coupling agents from both the qualitative and quantitative standpoints. Also the inaccuracy, low sensitivity, and high cost of such techniques has rendered them impractical in the analysis of the coupling agents used in many present day bead coating systems. The problems encountered with the testing procedures previously employed have been of special moment in the manufacture of coated beads on an industrial mass-production basis, with the result that the end product occasionally exhibited a coating that was too thin to provide the desired coupling action or that unnecessarily large quantities of the coupling agent were consumed.
In U.S. Pat. No. 3,837,806, Ritter et al disclosed a method of analyzing molecular layers of an amino silane coupling agent on glass spheres, in which a saturated solution of 1-chloro-2,4-dinitrobenzene in methyl alcohol is added to a sample of the spheres without substantial aggitation, and the materials are then heated to a temperature sufficient to react the 1-chloro-2,4-dinitrobenzene with the coupling agent. Ritter et al then compared the yellow color of the reaction product with successive color standards representative of known thicknesses of silane layers in order to determine the thickness of the layers on the spheres being analyzed.
The method disclosed by Ritter et al., however, is not sufficiently accurate for many desired analyses, particularly the detection of relatively low concentrations of coupling agents. At low concentrations of less than one molecular layer of coupling agent, the intensity of the yellow color of the product of the reaction between the coupling agent and 1-chloro-2, 4-dinitrobenzene is too low to be observed by visual examination. Thus a qualitative analysis using the method of Ritter et al. inaccurately fails to detect the existence of coupling agents when such coupling agents are present at concentrations of less than one molecular layer. The inability of the Ritter et al. test to detect small concentrations of coupling agent leads to an increased degree of inaccuracy even in quantitative measurements since small differences in the concentration of coupling agent may have too subtle an affect upon the color of the reaction product to be readily observable.
It is accordingly an object of the present invention to provide a method of analyzing molecular layers of coupling agents on non-porous inorganic substrates with greater precision than was possible under the prior art.
It is another object of the invention to provide a method of detecting low concentrations of less than one molecular layer of coupling agent on inorganic substrates.
It is another object of the invention to provide a method for determining the presence and thickness of amino functional group silane coupling agents on glass spheres and similar discrete particles.
A further object of the invention is to provide a method for determining the strength and extent of chemical bonding between coupling agents and inorganic substrates coated therewith.
Still another object of the invention is to provide a method of evaluating amino silane coupling agents on glass spheres which method may be quickly and easily performed in a rapid and inexpensive manner.